This project has continued to focus on studies of cell tropism of HIV infection. Our laboratory has developed a sensitive quantitative assay for HIV infection using CD4-positive HeLa cells. Most, but not all, HIV isolates infect these cells. Our recent experiments demonstrated that the block in infection by the nonpermissive isolates was due to lack of viral entry into the target cells. Using infectious molecular HIV clones of differing cell tropism recombinant HIV clones were synthesized, and it was found that a 120 base pair segment of V3 region of the viral envelope gene had a major influence on cell tropism. Interestingly, CD4-positive HeLa cells were infected at high efficiency by T lymphotropic virus strains, but were not infected by macrophage tropic strains. Thus, CD4-positive HeLa cells appear to be selective for T cell tropic HIV strains. Future studies are aimed at examining the cell tropism and V3 sequences of fresh HIV isolates from patients to determine the general relevance of these findings.